1. Field of the Invention
This invention relates to a real image mode variable magnification finder which is provided with a high-magnification zoom function and which is compact in size, and in particular, to a real image mode variable magnification finder optical system provided on an optical path independent of that of a photographic objective lens.
2. Description of the Related Art
In keeping with the tendency of camera lenses to high magnification and compact design, demands for high-magnification zoom and compaction on a finder optical system have increased. Some cameras are constructed to have the function of panoramic photography that enhances a wide-screen sensitivity of a photographic picture by laterally extending the range in which an image of an object falls on a film. In the finder of such a camera, there is a demand for a panoramic wide-screen sensitivity on the visual field of the finder, as well as for high-magnification zoom and compaction.
For the demand for high-magnification zoom on the finder optical system, if a lens unit of the objective system of the finder is designed to move in its optical path, the zoom function can be easily brought about. In this case, however, the traveling distance of the lens unit increases with increasing the magnification of the finder optical system, and the resulting overall length of the objective system becomes greater. This does not meet the demand of compaction. Also, the method is available that the zoom ratio of the finder optical system is improved by inserting an afocal converter in front of the first lens unit of the finder optical system (refer to Japanese Patent Preliminary Publication No. Hei 4-63313). However, this method, which needs at least two lenses, has the defect of increasing the overall length of the objective system.
As for the demand for compaction on the finder optical system, if the overall length of the finder optical system is intended for reduction, it is necessary to increase the refracting power of each lens. Consequently, the problem has arisen that aberrations deteriorate, and even though aspherical lenses are applied to the optical system, it is difficult to correct the aberrations. If the number of lenses is increased, the deteriorated aberrations may be corrected, but this will not bring about the reduction of the entire length of the optical system.
On the other hand, the panoramic wide-screen sensitivity in the finder visual field can be provided by increasing the magnification of the finder optical system in panoramic photography, and at the same time, by replacing a field frame for ordinary photography with a field frame for panoramic photography in which the field frame for ordinary photography is vertically reduced and horizontally enlarged. In this case also, however, the problem has been encountered that when a change in magnification is made by the movement of the lens unit, as mentioned above, the entire length of the optical system becomes greater. Thus, it is difficult to satisfy the limitations on the characteristics of the lenses of the finder optical system and on its mechanism at once, and to realize the high-magnification zoom and compaction.